The authors have been recently involved with the restoration of the various historical masonary structures as well as static and dynamic stability assessment of some natural monumental rock structures. Furthermore, dynamic limiting equilibrium methods (D-LEM) as well as numerical methods used for stability assessment of these structures. The outcomes of these studies are presented and their implications on the stability of historical masonary structures in Ryukyu Archipelago under dynamic loads such as those induced by earthquakes are discussed.
In blasting, cylindrical charge is generally applied in terms of energy efficiency. The dynamic fracturing in rock due to detonation of high-explosive involves quite fast process and extremely complex fracturing pattern. In addition, detailed fracturing process is generally not observable. Thus, blasting design tends to be based on empirical knowledge or law. It is also well known that fracturing process envisioned here depends on the applied pressure wave forms characterized by such as detonation property and amount of applied explosive. These in turn make the design optimization of blasting quite difficult and, even for the simplest blasting problem with a single free face, the fracturing mechanism has not been clarified yet. Therefore, it is of paramount importance to investigate the fracturing process due to blasting in detail for various types of applied pressure wave forms. For this purpose, application of numerical simulation is one of the most promising approaches. This paper proposed a method for the simulation of dynamic fracturing process through axisymmetric finite element formulation in which the initiation, propagation, branching and coalescence of fractures in heterogeneous rock can be analyzed. In particular, blasting a cylindrical charge through bottom priming in a cylindrical rock specimen was analyzed considering the difference of load configuration characterized by length of applied explosive. It was clarified that the resultant fracturing patterns were strongly dependent on the length of applied explosive. In addition, cross-shaped fractures occurring on the free face were successfully simulated by the proposed method, which were observed in the field-scale blasting with a cylindrical charge. Therefore, the applicability of the proposed method was validated and it can give a deep insight for understanding the dynamic fracturing process in rock due to blasting with a cylindrical charge.
Tunnels in Japan, which is especially constructed by the conventional tunneling method, often have voids behind the lining, that is, they have partial discontinuity between the lining and the ground. These defects may induce undesirable deformation and stress to the tunnel lining. However, there are only a few studies to evaluate the effect of a void to tunnel lining. In this paper, analytical solutions for a circular tunnel with a void behind the lining in the ground subject to external pressure are presented. The surrounding ground is taken as an infinite, homogeneous, isotropic, and linear elastic medium and the lining is treated as a linear elastic shell. Traction-free boundary condition is imposed on the place where there is a void behind the lining and no-slip boundary condition is imposed on the place where there is no void behind the lining. Numerical results show that the stress states of the lining is greatly affected by the presence of a void for isotropic compression and is a little affected for shear-type loading. The circumferential stress of the lining becomes higher in proportion to the extent of the void because of the decrease in the ground reaction. However, the circumferential stress of the lining becomes smaller when the extent of the void exceeds a certain range because the influence of the decrease in traction from the ground to lining is larger than that of the decrease in the ground reaction.
Two examples of inverse problem technique application to geomechanical tasks concerned with rock mass characterization were considered. An approach to estimating geometric parameters of underground voids formed in soil under natural or anthropogenic influence was offered. Irreversible deformation of soil is modeled using discrete element method. The formulated inverse problem on finding void geometry and occurrence depth by the data on configuration of the subsidence trough has unique solution. Based on geomechanical model of gas migration in block coal bed, the inverse problem was formulated for estimation of diffusion-capacity parameters (gas content, coefficients of diffusion and mass exchange) by the pressure measured in degassing borehole. The numerical analysis of cost function using synthetic input data revealed the necessity of the additional information on coal bed gas-kinetic properties for the inverse problem to be unambiguous solvable.
In the field of tectonic stress measurement, it is generally acknowledged that measuring the initial stress with high accuracy is difficult. We think the best approach to evaluate the initial rock stress is to carry out tests using multiple stress measurement techniques, to organize their mutual relationship and after reviewing the results, to performing a comprehensive evaluation, rather than reaching a conclusion based on a single measurement.
A hydraulic fracturing test has been carried out in-situ. Using the retrieved core, the DSCA method and a new technique that uses the Diametrical Core Deformation Analysis (DCDA) has been carried out.
Furthermore, using BHTV measurements, the breakout of the borehole has been confirmed and its occurrence has been interpreted.
In this study, the maximum and minimum principal stress, as well as the direction of the maximum principal stress in the horizontal plane, has been estimated based on a comprehensive evaluation of the above mentioned results.
Based on the each tests and the comprehensive evaluation, the direction of the maximum principal stress in the horizontal plane is estimated to be 85 degrees, the magnitude of the maximum principal stress at 1,700m depth to be between approximately 37 and 42MPa, and the magnitude of the minimum principal stress to be between approximately 34 and 38MPa. These results are consistent with the regional stress field derived from focal mechanism analysis.
The authors proposed a new rock mass classification named as Rock Mass Quality Rating (RMQR), which consists of six main input parameters, quantifies the state of rock mass and helps to estimate geomechanical properties (UCS, cohesion, friction angle, deformation modulus, Poisson’s ratio, tensile strength) of rock masses using a unified formula considering RMQR together with intrinsic geomechanical properties of intact rock. The comparisons of the empirical formula together with the values of constants were found to be quite consistent with the experimental results for data compiled from various rock engineering projects in Japan. Based on the databases of the authors, the application of the system was also extended to rock support selection for underground caverns and tunnels by considering the type of instability mode in relation to RMQR value. Some empirical relations, established between RMQR and dimensions of the elements of support systems, seem sufficient for many engineering applications and act as guidelines.
The objective of this study is to determine rock mass strength and deformability in the laboratory by simulating joints in sandstone specimens. The results are used to assess the predictive capability of rock mass strength criteria developed by Hoek-Brown, Ramamurthy-Arora, Yudhbir and Sheorey. Empirical criteria developed by Goodman, Yoshinaka and Yamabe and Ramamurthy criteria are used to predict the deformation modulus of rock mass. Triaxial compressive strength tests have been performed on cubical sandstone specimens with nominal dimensions of 60×60×60 mm3 and 80×80×80 mm3.A true triaxial load frame is used to apply confining pressures up to 12 MPa. The joints are simulated by saw-cut surface. The compressive strengths and deformation modulus of rock specimens decrease exponentially with increasing joint frequency. Under the same joint frequency the deformation modulus tends to increase with confining pressure. When the major principal stress is normal to the joint planes the specimen strength is greatest and the deformation modulus is lowest. Hoek-Brown strength criterion can effectively describe the specimen strengths under all configurations. The m and s parameters decrease rapidly with increasing joint frequency and joint set number. Ramamurthy criterion can best describe the deformation modulus of rock specimens under all configurations.
In-situ torsional shear test for rock masses, which was proposed by the authors, can be used to obtain mechanical properties of anisotropic rocks by evaluating stress distributions on the top of hollow cylindrical specimen by separate bi-directional load cells. For developing this testing method, transversely isotropic elastic constitutive model is assumed, and elastic solutions under an isotropic consolidation and a torsional shearing are calculated. The results demonstrated the following three conclusions: the directions of plane of isotropy can be determined by directions of three principal strains calculated from six strain components measured on the lateral side of the specimen during isotropic consolidation; stress distributions change in a trigonometric function to the circumferential direction during torsional shearing because anisotropic specimen does not necessarily satisfy an axi-symmetry condition; five elastic parameters of transversely isotropic rocks can be obtained by analysis of test results using both strain responses during isotropic stress consolidation and stress distributions during torsional shearing.
Grouting is performed to mechanically improve the deformation and strength characteristics of bedrock by filling cracks with cement so that the entire bedrock is integrated and homogenized. For verifying the improvement, however, only permeability is currently considered because it is difficult to track other mechanical properties. For this reason, today dam foundation design cannot fully account for improvements, even though many construction activities are carried out to improve the mechanical properties of dam foundations.
Against this background, we compared the results of borehole loading tests and indoor shear tests,`before and after grouting. Based on the test results, the mechanical improvement of bedrock by grouting was characterized in order to develop a way to account for the mechanical improvement of bedrock in dam foundation design.
The GIN model for rock fractures grouting is widely used in practice. The element shared by the practitioners is the GIN envelope. However, different methods are adopted for approaching the GIN envelope during the injection process of a cement mix. Some practitioners stop immediately when reaching the envelope and others follow it for a while. There is no general rule for when and where on the envelope grouting should be stopped. A parameterization of the penetrability on the envelope is provided in this paper. For the parameterization, the grout rheology, the fracture’s thickness, the borehole radius and the target are taken into consideration. This allows to define a penetrability limit beyond which the grout process may be stopped. The penetrability is then extended to the case many fractures are injected simultaneously. Numerical simulations are used for graphical illustrations.